General Tips for Sending to IONOS Mail Servers

In this article, we have compiled information and tips about the delivery of emails to IONOS mail servers for you. This can be helpful, for example, if you are having trouble sending emails to someone using IONOS mail servers but cannot figure out what the issue may be.

Tips for Users

I have set up my email program, but I cannot send emails to IONOS.
Please check your email program settings against your provider's recommendations. Also, take note of the error messages or try to deliver the email in another way.

What is the maximum size of attachments I can send to IONOS users?
The maximum size of an allowable email is 100 MB. Please note, however, that the file size must not exceed 72 MB for encrypted messages.
In principle, sending emails with correspondingly large attachments is not recommended. Under certain circumstances this can lead to longer loading times or various error messages.

Why are my emails not being delivered?
If an email cannot be delivered, IONOS always sends an error message to the original sender. Please check the list of error messages to see why the email is not sent.


My emails will not be delivered. Why don't I get an error message?
If an email cannot be delivered, IONOS always sends an error message to the original sender. Please check whether the error message was marked as spam or moved to another folder by a filter rule. Please also check whether the email system you use forwards error messages to the users.

Tips for Administrators

Why are my emails delayed?
Since email is an asynchronous medium, we, like all other providers, cannot guarantee immediate delivery. However, we work with a large team of specialists to enable problem-free correspondence via email.


Why are my emails considered spam?
The IONOS spam filter systems consist of numerous different IP-based and content-based filter modules. This may result in an email being misidentified as spam. Please send us the extended header of such an email. Please note that emails that are sorted into the folder "Unknown" have already been rated with a low spam probability.


How can I ensure that my emails are not rated as spam?
Please note our recommendations for sending emails.


There was a problem with my systems, but it has already been fixed. Why are emails still not accepted from me?
If our systems detect that spam is being sent from certain servers, the compromised systems are blocked on an IP basis. Please use this contact form so that we can activate your systems manually.


How do I get IONOS on the whitelist?
Doesn't IONOS offer this. Please note our recommendations for sending emails. If you observe these, no whitelisting is necessary.


IONOS greylisting?
We take the liberty of delaying or completely rejecting the sending of emails from an IP address in the case of unique features that indicate servers sending spam. Please note our recommendations for sending emails. If you consider these references, a delayed acceptance of your emails is to be excluded.


Why is no connection from my server to the IONOS systems allowed?
Please check whether the IP address of your email server appears on one of the established RBL lists. You can check this fact here, for example. In such a case, please contact the operator of the list directly.


Also check the log files of your server from the first occurrence of an error.


Check whether the standards defined in the RFCs have been adhered to. These include, for example, specifying a syntactically correct sender address, using a HELO that can be resolved backwards to the IP address, and setting up a reverse DNS entry (FQDN) for the IP address of your email server.


Unauthorized persons may have sent emails via your systems. Secure your systems against unauthorized use.

If you send newsletters or mass mails, please check the delivery rate and remove any email addresses that no longer exist from your databases. Only send emails to recipients who have explicitly given you permission to send your emails.


Please contact us in order to clarify the facts or to activate your servers. Please send us meaningful log file extracts and a description of the error that has occurred, including the error messages. Please use our contact form.